Lab: Flame Tests

Objective

In this lab students will learn about atomic energy levels, emission spectroscopy, and flame tests for element identification.

Overview

Students will observe small samples of chloride salts of different metals. These will be placed into a flame in order to observe the colors produced. These colors come from the excitation of electrons which then resume their ground states by emitting light of very specific colors.

Background

The electrons in an atom occupy different energy levels. When all of the electrons are at the lowest possible energy level they are said to be in the ground state. In this state electrons are as close to the atom’s nucleus as they are allowed to be. When they are there, they have their lowest possible potential energy. Just as a ball sitting on the ground has its lowest potential energy, electrons which are close to the nucleus are similarly low in potential energy.

Electrons do not always stay in the ground state. Sometimes they can be promoted to an electron shell with a higher potential energy. This can happen in two ways. First, the electron can absorb a photon of just the right amount of energy to move it from one quantum shell to another. Second, when atoms are heated in a flame or energized with electricity their electrons can gain energy. This promotes them to the higher-energy shell. When an electron is in a higher-energy shell it is said to be in an excited state. Excited states are situation where one or more electrons within an atom are at a higher potential energy than they would be in the ground state. There are many possible excited states for atoms.

Electrons in excited states do not stay in them for very long. When electrons lose their energy they do so by emitting light. In this way, excess potential energy is transformed into the energy of light. Each time a single electron drops from higher to lower potential energy, a particle called a photon is produced. Photons are particles with energy but no mass. Their energy is inversely proportional to the wavelength of the light (this can be expressed in the proportion, E = hc/λ, where λ means the wavelength). The photons emitted precisely match the quantum energy difference between the excited state and the ground state.

A salt is a type of compound that include a metal ion and a non-metal ion. Sodium chloride (NaCl) is the most familiar example of a salt but others include calcium chloride(CaCl₂) and copper(II) chloride (CuCl₂). These compounds look very much the same to the unaided eye. For example both sodium chloride and calcium chloride are colorless crystals and as a fine powder they are white. In order to identify which salt you have, you have to do a test on the material. A traditional method for identifying elements in compounds is called a flame test.

In flame tests salts that are dissolved in water are evaporated using a hot flame. In the flame the atoms become excited and produce a characteristic color. This happens when the atoms’ excited electrons fall back down to the ground state. The color we see depends on the difference in energy between the excited and ground state. For some atoms this is a large difference and for others it is a smaller difference. The color can be used to tell if the energy difference is large or small. The purple end of the spectrum has the smallest wavelength and so the energy of that light is the largest. The red end of the spectrum has the largest wavelength and so that color represents the smallest energy changes. Remember the order of colors in a rainbow is ROY G BIV (red orange yellow green blue indigo violet). Energy changes can be judged against the spectrum because as you go from red to violet the energy change that made the color gets larger. Different atoms have different possible excited states, which vary in their distance from the atom’s ground state. Each element has a unique set of possible excited states. Because of this it is possible to identify an element by doing a flame test and recording the observed color. Every element has a unique flame test color.

It is a traditional art of the chemistry laboratory to use these colors to identify specimens of compounds that contain unknown metals. This is no longer a common laboratory technique though highly advanced technologies use the same production of light to identify elements and to measure how much is present. The production of light in a flame is still used outside the lab, though. It is put to use by practitioners of the art of fireworks manufacture. By including different metal salts, or mixtures of metal salts, in the exploding shell of a firework, these artists can produce beautiful displays in nearly all the colors of the rainbow.

Light is a kind of wave, an electromagnetic wave. Our ability to perceive color depends on the different wavelengths of light. If you observe a rainbow or use a prism to split white light then you are looking at how our eyes and brain work together to give us the sensation of the colors of the rainbow. Because we see different ranges of the visible spectrum as different colors it’s possible to roughly assign a color we see to a particular wavelength of light. It is not a perfect match, but it’s a good start and is good enough for now. Take a look at the chart at right. It details the colors people in our culture usually assign to the rainbow and gives each color a range of wavelengths in units of nanometers. A nanometer (nm) is a billionth of a meter (1 m = 1 × 10⁹ nm). Each color can be assigned a ‘representative wavelength’. You will use these to identify colors in this activity.

Materials

Safety

• Wear goggles or risk sitting out the lab
• Treat all chemicals in this lab as toxic. Do not touch any of them with your bare hands.
• Wash well with water immediately if you touch chemicals accidentally
• Use caution with the burner
  • Do not leave burner unattended
  • Place burner near middle of lab bench
  • Tie back long hair
  • Do not wear baggy clothing in the lab
  • Hot objects look like cold objects: be cautious!
• CuCl₂ (Copper(II) Chloride) is highly toxic by ingestion; avoid contact with eyes, skin and mucus membranes.
• LiCl (Lithium Chloride) is moderately toxic by ingestion; avoid contact with eyes, skin and mucus membranes.
• Wash your hands with soap and water after you complete the day’s lab work, even if you didn’t touch any chemicals directly

Procedure

Remember to record your observations in your lab notebook or on a piece of paper in your binder before you leave class. When making observations be sure to use all of your senses except taste. Never taste anything in the chemistry lab. Chances are good you will regret it if you do.

1. Each group member must record information in a neat table with the following columns. Make this table before you even turn on the gas.

   Ionic Compound Formula	Cation (positive ion)	Anion (negative ion)	Color Observed	Representative Wavelength (nm)	Element Responsible for Color

   Do not fill in the final column: “Element Responsible for Color”. You will fill in this column as part of the post-lab questions. To help identify ions, you may use your Ions Reference Sheet. Choose whatever color is closest from the table of representative wavelengths in the introduction of this lab.
2. Clean the inoculation loop by swirling it gently in some clean water. Then, once you light the burner, heat the loop until it glows red hot. Note that the wire alone does change the color of the flame. Ignore this color when you observe the flame test colors of the various samples.
3. Light and adjust your Bunsen burner. Be sure to clean your loop carefully. Do not leave the loop in the flame too long as it can cause the loop to degrade and break.
4. Each of the samples you will use are already available in labeled beakers. You only need one of these at a time. Use the inoculation loop that is next to each beaker so that each loop is only used with one sample.
5. To do a flame test with each metal salt get a film of the solution of a salt inside the loop and bring it into the flame. Try the edge, the top, and the center of the flame. Repeat the dip into the salt solution as often as necessary to see the flame test color. Be sure not to over-heat the loop.
6. Carefully note the color of each metal salt when it is put in the flame. Use the chart on the previous page to estimate the approximate wavelength of the color you see. Use the Representative Wavelength values. Record all data in the table you made earlier.
7. Your teacher has prepared two solutions with two of the metal salts. They are labeled Unknown 1 and Unknown 2. Test these in order to determine what they are. The unknowns each correspond to one of the salts you have observed. By comparing the unknowns with samples of a known identity you will be able to identify them.
8. The labeled sample beakers may be used for another class. Try to leave things neat and tidy and keep the wire loop with its designated sample.
9. If your class is the last to use the sample, clean out the beaker using the method recommended by your instructor (hazardous wastes must be disposed of properly). Usually, all leftover solutions will be collected in designated waste containers for hazardous waste disposal.
10. Wash all equipment carefully and thoroughly using the tub of soapy water provided. By gently scrubbing the beakers with a brush and the soapy water you will be able to wash off the labels you put on them. Please do so!

Grading

For this lab you must turn in the following items:

1. Your data table recording flame test colors and wavelengths
2. Answers to the following questions
3. A one-page essay about how fireworks are made, concentrating on how they produce different colors but including details of their basic construction and operation

Questions

1. According to the introduction on this lab handout, what is the chemical definition of a salt?
2. According to the introduction on this lab handout, what is the ground state of an atom?
3. According to the introduction on this lab handout, what is an excited state of an atom?
4. Based on your reading of the introduction, what has to happen within an atom (that is, what happens to the electrons in an atom) before it can produce light?
5. Based on your reading of the introduction, why do atoms need to be heated in a hot flame or subjected to high-voltage electricity before they will produce any light?
6. Based on your reading of the introduction, which color represents a larger energy change within an atom: blue or orange? Explain.
7. Determine the element that is responsible for the color of each flame test by considering the following questions. Answer each with appropriately formatted chemical formulas and complete sentences as necessary.
   1. What are the names and formulas of all of the compounds that contain the chloride ion (Cl^–)?
   2. Do any compounds containing the chloride ion (Cl^–) have the same color? Explain.
   3. Is it the cation or the anion that is responsible for the color of a flame test?
8. After completing the questions above, fill in the final column on your table.

Web Links for Part III of the Lab